For various reasons it may be necessary to take representative samples of non-homogeneous fluids in order to determine the concentration of the components in the fluids. The sampling of a fluid is normally combined with the transport of the fluid through a pipeline, being one of the most important transporting means in many industries handling large quantities of fluids.
One example where the sampling of fluids has become increasingly important in the recent years is found in the oil industry. Sampling of crude oil for determining the net amount of oil has to be performed very accurately in connection with the metering of crude oil for royalty purposes. The knowledge of the correct amount of hydrocarbons in the available crude oil will further enable refinery operation management to draw up more accurate mass balances, so that the various refinery operations may be carried out in the most efficient manner.
A large variety of methods are known for sampling fluids; however, all these methods suffer from being not accurate enough when non-homogeneous fluids are to be sampled. The main reason for this inaccuracy consists herein that the samples which are taken from a fluid stream in a pipeline in the known methods are not representative for the bulk of fluid but only for that part of the fluid passing the sample point at the moment the samples are taken.
A problem in sampling crude oil to determine the content of water and bottom sediments in the oil, is formed by the fact that the water and other contaminations are hardly ever uniformly distributed in the oil. When crude oil is stored in a tank, the water and bottom sediments will settle down in the bottom part of the tank. When, for example, crude oil is unloaded from a tanker and transferred through a pipeline the amount of water and bottom sediments in the crude oil will exhibit large fluctuations. When the unloading of a tank in the tanker is in an advanced stage the crude oil passing through the pipeline will contain no or only small amounts of water dispersed therein. However, when the unloading of a further tank is started the amount of water in the crude oil will suddenly increase, and even slugs of water ray pass through the pipeline. Samples taken from the first part of the crude oil unloaded from a tanker may therefore totally differ from samples taken from the crude oil during the last part of the unloading. None of these samples nor the combined samples will necessarily be representative for the total mass of crude oil unloaded from the tanker. Further large variations in throughput may occur during the unloading process, causing further inaccuracies in the known sampling methods.